Renewable energy, harvested from natural phenomena such as sunlight and wind, has replaced conventional fuels in areas of energy service and power generation. Electrical power devices, such as semiconductors, convert energy obtained from renewable energy sources, such as solar power and wind power, from dc or variable frequency ac to utility (grid) frequency. Conversion is generally achieved by switching a plurality of semiconductors on and off at high frequency to synthesize sine wave voltages via pulse width modulation (PWM) techniques.
Three-terminal semiconductors, such as insulated-gate bipolar transistor (IGBTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), and thyristors are commonly used as switching devices in electrical applications. MOSFETs, used primarily in low voltage applications, may increase the current rating of a switch by placing several devices in parallel. Thyristors, used primarily in high voltage applications, require a finite time delay after the current in the thyristor has extinguished before the anode can again be positively biased and retain the thyristor in the off-state. IGBTs combine the performance in the on-state in high voltage devices with high impedance, thus making it a popular choice in power generation applications.
Transitioning each IGBT from the on-state to the off-state and vice versa results in heat production, as power is dissipated within each IGBT during switching events. The higher the current being switched on or off, the more losses, therefore the more heat. As current increases, the conduction losses within each IGBT also increase. Just as the power from renewable energy sources can often vary throughout a day, so can losses dissipated in the IGBTs. Therefore, the temperature of the IGBTs vary with changes in the output of the renewable energy sources. The thermal cycling, which may occur over minutes or hours, can result in mechanical stress and increased rates of IGBT failure over time due to material fatigue.
The life of an IGBT can be reduced due to thermal cycles. Thermal cycles, specifically in the baseplate and the bond wire of the switch, among others factors, results in device fatigue. Extreme changes between high and low temperatures decrease IGBT life more than small changes between high and low temperatures.
Power within alternating current (AC) system is equal to the product of voltage, current, and power factor. Apparent power can be classified as two components, one in phase with the voltage (real power) and the other 90 degrees out of phase with the voltage (reactive power). Real power delivered to a grid is primarily a function of how much power the renewable energy source produces (e.g., proportional to wind speed of the wind or strength of the sun). Reactive power is delivered to control voltage to meet energy storage requirements for system reliability (e.g., of inductances and capacitances in transmission lines, motors, and other devices).
Prior attempts to increase fatigue tolerance within semiconductors are known in the art. For example, many conventional approaches include variations of altering the surfaces of the semiconductor to prolong the life of the device. This approach, however, does not prevent cracks from expanding to critical sizes, for example, which can ultimately lead to fractures.